The axial fuel bundle configuration of the RBWR-AC is shown in Fig. 14.5. The axial configuration is the parfait core, where an internal blanket (520 mm) of depleted uranium oxide is placed between two fissile zones (upper, 280 mm; lower, 193 mm). The upper and lower blankets (70 and 280 mm) are attached above and below the upper and lower fissile zones, respectively.

The neutron absorber zones are placed above and below the fuel zone (fissile and blanket) to increase the margin to maintain the negative void reactivity coefficient. The upper neutron absorber zone is composed of the neutron absorber rods placed between the plenums, which are connected to the fuel rods. The neutron absorber rods are filled with B4C pellets in a sealed tube with an outside diameter of 7.7 mm.

Plenum and holder

500mm

Plenum and neutron absorberrod 500mm

Plenum 300mm Upper blanket "0mm

Upper fissile zone

2S0mm

Intemalblanket

520mm

Lower fissile zone

193mm

Lowerblanket 2S0mm Lower neutron absorption zone ‘0mm

Each neutron absorber rod is attached to support rods fixed with the upper tie-plate of the fuel bundle. The neutron absorber rods are installed in a ratio of one per one fuel rod. Each neutron absorber rod is 500 mm long, and the distance between the upper end of the fuel zone and the lower end of the neutron absorber rod is 300 mm. The lower neutron absorber zone is composed of B4C pellets filled in the fuel cladding. The length of the lower neutron absorber zone is 70 mm.

Figure 14.6 shows a horizontal cross-sectional view of the configuration of the RBWR-AC fuel bundle and its fissile Pu enrichment distribution. The lattice pitches of the fuel bundles are 199.2 mm on the side with the control rod and 194.7 mm on the side without it. The channel box of the fuel bundle is hexagonal with an inner width of 189.1 mm, and its wall thickness is 2.4 mm. The control rod is 6.5 mm thick, and the gap between the rod outer surface and the channel box is 1.6 mm on each side; the gap between channel boxes on the side without the control rod is

0. 8 mm.

The fuel rod gap and pitch are 1.3 and 11.4 mm, respectively. For the equilib­rium core of the RBWR-AC, the bundle-averaged fissile plutonium enrichment is

15.7 wt% for the upper fissile zone (Fig. 14.6a) and 20.1 wt% for the lower fissile zone (Fig. 14.6b). Both the upper and lower fissile zones utilize five different fissile Pu enrichments.

The main core specifications and performance values of the RBWR-AC in the equilibrium core are shown in Table 14.2. The core coolant flow is 2.6 x 104 t/h at a subcooling temperature of 5 K at the entrance and has a steam quality of 35 w/o at the core exit. The void fraction of core coolant is about 30 % at the bottom of the lower fissile zone because of heating in the lower blanket; it reaches 80 % at the top of the core. A breeding ratio of 1.01 is achievable under a 45 GWd/t exposure

Average fissile Puenrichment

15.7 wt%

Number of fuel rods 271

Fuel rod diameter 10.1mm

Fuel rod gap 1.3mm

Thickness of control rod 6.5mm

Average fissile Puenrichment
20.1 wt%

Number of fuel rods 271

Fuel rod diameter 10.1 mm

Fuel rod gap 1.3 mm

Thickness of control rod 6.5 mm

Lower fissile zone

Table 14.2 Core specifications and performance values [3] Item RBWR-AC RBWR-TB RBWR-TB2 Core height (mm) 1,343 993 1,025 Fuel rod diameter (mm) 10.1 7.4 7.2 Fuel rod pitch (mm) 11.4 9.4 9.4 Fuel rod gap (mm) 1.3 2.0 2.2 Pellet diameter (mm) 8.7 6.1 6.0 Number of fuel rods 271 397 397 Coolant flow rate (t/h) 2.6 x 104 3.8 x 104 2.4 x 104 Core exit quality (%) 35 21 36 Void fraction (%) 53 42 56 Pressure drop (MPa) 0.14 0.19 0.06 HM inventory (t) 144 77 76 Puf/HM in fissile zone (wt%) 15.7/20.1 13.9 25 Puf inventory (t) 9.0 4.5 8.3 Burn-up (GWd/t) 45 55 65 MLHGR (kW/m) 47 47 47 MCPR 1.28 1.3 1.28 Void reactivity coefficient (Ak/k/%void) —2.4 x 10—4 —2 x 10—4 —4 x 10—4 Breeding ratio 1.01 — — TRU fission efficiency (%) — 51 45

averaged with the upper, internal, and lower blankets. Here the breeding ratio is defined as the number of atoms of fissile plutonium left in the discharged fuel bundles per fissile plutonium loaded in the initial charged fuel bundles.

The loading pattern of the fuel bundles in the equilibrium core adopts zone loading with the reflective boundary condition of 60 ° in the azimuthal direction. After the control rod scheduling is done, the radial power peaking factor is about 1.2 and the axial power peaking factor is about 1.8, including the blanket zones, which results in the minimum critical power ratio of 1.3 and the maximum linear heat­generating rate of 47 kW/m.

The RBWR-AC has a void reactivity coefficient of —2.4 x 10—4 Ak/k/%void, which is comparable with that of the current BWR, about —7 x 10—4 Ak/k/%void.